Many radio communication devices, such as two-way portable transceivers are equipped with a squelch circuit to disable audio output when a communication channel being monitored has unacceptable levels of noise. In a typical radio, a communication channel is monitored to detect a communication signal. For example, a communication signal may represent voice communication. If the detected signal is above a particular threshold, the radio's audio output is unmuted to provide a decoded output of the detected signal. When the communication signal is no longer detected, the radio's audio output is muted to avoid outputs attributable to noise on the communication channel. Additionally, during typical communication sessions, a communication signal level may become weak, such that a signal level to noise ratio (S/N) falls below a particular threshold, thereby resulting in the muting of the radio's audio output.
Abrupt muting of a radio's audio output may not be ideal in certain communication environments. For example, if the radio is being operated under weak signal conditions, the signal to noise ratio may periodically drop below the mute threshold value which may result in undesirable muting and unmuting operations. For example, fluctuations in detected signal level may result in the radio muting prematurely. Premature muting may result in portions of the communication signal being lost. Yet, the muting threshold level must be set to disable audio output under noisy channel conditions, or when a communication signal is no longer present on the channel.
One solution to the problem of premature or undesirable muting of a radio's audio output is a provision of a variable time delay for mute operations. A variable time delay may allow a very weak signal to fluctuate without premature muting. Variable time delay muting may be provided by a squelch tail circuit. A squelch tail circuit usually varies the delay time for muting operations depending on a detected signal strength. For example, a squelch tail circuit may cause the abrupt muting of a radio's audio output upon detection of a loss of signal immediately following the detection of a strong signal. On the other hand, upon detection of a weak signal, the squelch tail circuit may provide a relatively long time delay before muting, the assumption being that periodic signal loss may be resulting from fluctuations of a weak signal.
In a typical squelch tail circuit of the prior art, a comparison is made between the strength of a communication signal, such as an audio signal, and the level of noise detected with that signal, to govern the activation of audio output muting. The provision of a squelch tail circuit typically requires dedicated circuitry that measures S/N for comparisons against a threshold. The measurement of S/N is quite common in an analog signal processing environment.
Many modem radios now employ digital signaling to effect communications on a communication channel. A substantial portion of analog signal processing has been carried over to systems employing digital signal processing. For example, U.S. Pat. No. 5,303,407 issued to Juergensen, et al., on Apr. 12, 1994, for A DIGITAL SQUELCH TAIL CIRCUIT FOR TWO-WAY RADIO COMMUNICATION SYSTEMS, describes a squelch tail circuit that uses a digital timer to provide a time delay based upon a feedback comparisons to determine time delay periods for muting audio output corresponding to weak and strong signals. In another example, described in U.S. Pat. No. 5,023,940, issued to Johnson, et al., on Jun. 11, 1991, for A LOW-POWER DSP SQUELCH, a statistically evaluation of a received digital signal is used to determine whether the communication channel contains signal or noise, and this information is used in determining operation of a squelch circuit. In a third example, such as that described in U.S. Pat. No. 5,303,408, issued to Ghomeshi, et al., on Apr. 12, 1994, for A SQUELCH DETECTOR, a communication device includes a digital signal processor to measure the level of signal energy in various segments of frequency spectrum of a received signal to dynamically establish a squelch threshold. In all three examples, a measurement or approximation of signal energy and noise energy is used in determining when to squelch.
Digital communication systems can provide improved communication efficiency by applying a substantial amount of logic to the processing of received signals. Such additional signal processing capability is not fully utilized in many of today's communication systems that attempt to remove unwanted noise from audio outputs of received digital signals. Therefore, it is desirable to provide an improved digital squelch tail system that more fully utilizes the signal processing capabilities associated with digital signals.